Power Supplies and Pump Motors

I would use a set of auto transformers to bring the voltage down. Its the simplest way I can think of. Being they only transform the 100 volt difference and not the full power of each leg of the three phase system their size is smaller and cost is much lower.

There are many transformer suppliers and dealers online that carry autotransformers that are designed just for your voltage difference.

Contact a few and tell them what your pump voltage and amperage requirements are and they will tell you exactly what size of autotransformer you need.

WRONG...

You cannot do this, because if you lower the voltage without lowerng the frequency at the same rate you will have a different V/Hz ratio and the motor will over heat.

I am not sure how you figure thats wrong. Where are you getting your information?

I worked at a pasta plant years ago and all of the motors on the machinery were European specs. (50 Hz, 380 volt rated) All they did was use was simple 480/380 auto transformers to bring down the voltage to them and change the pulley ratios on the stuff that needed specific RPM rates. The majority of the motors were original and had been in service for over 20 years and still worked just fine.

Around here motors work just fine if the frequency is slightly higher and the voltage stays the same. I have many 230/380 volt 50 Hz motors myself that I run on 240 volts 60 Hz with no problems at all.

Standard motors I have and use typically say 50/60 Hz right on the tag and have the amps and RPM ratings for both But only one voltage rating for both frequencies.

I would speculate that from my own hands on experiences that it is you who's wrong or at least greatly misinformed and should consider doing some honest research into what you are saying before shooting your mouth off about who's correct and who's not.

Just something to think about.

When you change the V/Hz ratio, you change the saturation point of the rotor and stator. So by lowering the voltage without changing the frequency, you are going from a V/Hz ratio of 7.6:1 (380/50) to 6.33:1 (380/60). This has the exact same effect as if you had dropped the voltage at 50Hz to 317V: the torque output of the motor will drop by the square of the voltage reduction. So at the equivalent of 83% voltage, the torque of the motor will drop to 69% of the motor's rated output. This drops your 3kW motor shaft power down to about 2kW. At the same time, the speed will increase by 20% which effectively regains SOME of the shaft power, so you have 69% torque at 120% speed, and your shaft power has gone from 3kW to roughly 2.5kW. But on a centrifugal pump, now you have a bigger problem. Power requirements on a centrifugal pump vary by the cube of the flow, and flow changes proportionally to speed. So at 120% speed, the flow will increase to 120% and the power REQUIRED by the pump will increase to 173% of what it was. So now the motor shaft power has dropped to 2.5kW, AND at the same time, the pump is needing potentially 5.2kW! Even if the pump motor was originally over sized by lets say 20%, so it only needed 2.4kW out of the 3kW used, he STILL needs 4.1kW from a motor that is now only capable of delivering 2.5kW.

So no matter how you cut it, the motor is still either too small or he needs to just run it at 480V and hope it was a little over sized, or reduce the flow with a valve, or use a VFD. The only thing he CANNOT do is just use a transformer by itself.

So why did it work for you in the past? Most likely because they were not pumps,AND they were generously sized in the original design. So although all of your motors dropped in output kW, your loading still fell within a level the motors could tolerate. But such is the danger of anecdotal information. It MIGHT work under SOME circumstances, but you have to do the math in advance to know if it is safe to try.

Bottom line you were lucky, but luck is not an accepted engineering principal.

I never engineered it. The pasta plant was set up that way a decade or two before I ever knew it existed.

Its just a common thing I see fairly often with equipment from different parts of the world being used in American applications. I have seen many pumps set up that way too. Engineered correctly our not its still a common occurrence and it apparently works more often than not. I cant say it all right or all wrong, its just gets done that way fairly often and it apparently is not a big issue since the official electrical inspectors that sign off on the motors, or systems in general, being ran that way don't seem to have any issues with it.

To me this is nothing more than an idealistic engineered concept VS the what gets used in the real world application difference. I stand by my I see it done quite often therefore it must work the majority of the time reasoning. If the motor burns up eventually so what? It likely needed to be changed over anyway. If it doesn't burn up don't mess with it.

As the ratio f/v seems good the motor will work pefectly (only quicker) but would the pump construction withstand the increased speed by about 20% ? Then, maybe, worth buying a VFD to keep the speed down to that specified by the manufacturer of the pump.

The V/F ratio is close, but still higher than original spec. Concern about over-voltage may be correct. With your 3kW rating, a solid state VFD should be a cost effective solution.

As you noted, the V/Hz ratio is close enough, voltage can vary by +- 10%. But the problem, alluded to by an earlier post, is that the motor will turn faster. Because motor power is a function of speed and torque, and the speed has increased, the motor output will increase proportionally. BUT in a pump, speed relates to flow, and power consumption varies by the cube of the flow. So if your motor is running at 120% speed (60/50) then the flow will increase accordingly and the power demand on the motor will increase by 1.2³ , or 178% of the power at 50Hz. In other words, your 3kW motor will increase its output power by 20% to 3.6kW, but the LOAD demand from the motor may increase to 5.4kW!

This assumes that flow is left unrestricted. If you have some way of restricting flow to what the maximum design was, then it will be OK, but if not, your motor will over load.

The question here is HOW 380 VOLTS RATED MOTOR COULD BEHAVE WHILE WE FEED 480 VOLTS 50/60HZ.

WHILE WE USE VFD ,THE DC LINK CAPACITOR VOLTAGE RATING COULD BE INCREASED TO 1000 VOLTS RATING ie four capacitors two in seriesand these two parellel of 500 volts capacitors of suitable values of capacitance.

in vfd control automatically maintains constants of v/f ,hence required speed ,no hastles of over load.

gopalraju

Absolutely you can't hook up your pump motor into different voltages. The simple logic is it can only plug into the right voltage supply as per in the nameplate voltage rating. Take note that varrying the supply voltage will change of starting torque and ambient temp. of the motor as well. Simple analysis is that when the voltage increases, the RPM also increases more than the rated RPM in the nameplate that will cause to overun/rugged and carbonized the commutator. Again, when the voltage supply decreases, it increases the torque and current draws of the motor causes the increase of ambient temperature more than the rated amb. temp at nameplate and start burning the motor windings. If there is no other way to re-connect windings( you did not tell us exactly if your motor is 6 or nine leads where you have an option to re-connect to match it into available supply voltage). Use the auto-transformer rated 480/380/220V 3-phase, available commercial size 5kva, 10kva up, brands are Manumag, G.E., Westing House, Cutler Hammer, ABB, etc... kindly check various manufactures/suppliers to satisfy your technical requirements)

deare,

as you summarise from the opinions received,

you can't, since

1. motor is not rated for voltage > 10%

2. bearings are not rated for speeds > 10%

3. Loading conditions of cubic variations in power does not allow your motor to get over loaded, shaft may get damaged.

4. electronic components like capacirtors may have to be up graded by 100%

so, better use auto transformer to lower the voltage & operate the pump.

I respectfully mildly disagree with #2, and #4 and the conclusion.

#2: While the bearings MIGHT not be rated for a higher speed, 20% higher speed is not likely a factor.Most bearing mfrs design for a world wide market so it's doubtful that the bearings chosen by the motor mfr would be rated to such a narrow speed range.

#4: Same for the caps (assuming we are speaking of the VFD components). Most VFDs are designed around what is called a "400V Class", which covers an input voltage range of 380 to 480 +- 10%. I don't know of any 380V rated VFD that would not work on a 480V supply.

Your conclusion that a transformer is the best solution is not correct. See my response to that earlier suggestion above.

Why are the "If it worked once it must be OK", brigade who for some reason look down on any theoretical approach, probobly because its beyond their tiny minds, Aways the most vocal? If I hear the expression "in the real world" or equivilent again I will scream!

Keep up the good work nice to see you can rise above the personal insults that the non engineers always seem to sink to.

Well start screaming your head off at the world because the majority of the world are not engineers and are often in positions where they have to make due with what does work idealistically or not.

Ideal mathematical formulas are great for theory but often fall far short when actually applied to a real working conditions.

My take on why it works is this. The OP wants to run a 50 HZ rated motor on 60 Hz not the other way around. That seems to have been overlooked by almost all so far.

Running a 50 HZ rated motor at 60 Hz works just fine in most realistic circumstances and here is why. Since the frequency is higher than its original spec that does give it more peak RPMs. However the inductance of the motor will also change its inherent torque and counter EMF characteristics slightly. That means that when the same input voltage is applied the motor does spin faster but the slightly higher reluctance values caused by the higher frequency will work like a current limiter to a small degree because the original V/F ratio make the new input voltage a bit lower than its idealistic V/F ratio would suggest for it.

That slightly less than ideal input voltage for a 60 Hz based formula makes the rotor slip a bit more and not actually reach the full equivalent of the pole to frequency ratio a typical 60 Hz V/F ratio designed motor does. That will put the typical RPM output between the typical 50 Hz motor rate and the typical 60 Hz motor rate but still favoring the 60 rate though at light loads but dropping near to the old 50 Hz speed when loaded down.

The added slip rates at load will change the new V/F reluctance to a lower value allowing the input current to rise back up to closer to the original name plate spec.

That is saying even though the frequency is higher if the input voltage is kept the same the running amps input will still be typically near the same when applied to the real world working load condition. The higher frequency tends to pickup a slight gain in winding efficiency by trading in the apparent power amps for real power energy transfer. The input wattage will likely be slightly higher and the apparent power value slightly lower but the overall amps draw of the motor didn't change by any significant amount. The total amp draw still stays within the working current parameters of the windings themselves even though the true power and apparent power number are slightly different.

The V/F ratio is a ideal number but if the voltage is dropped slightly below that ideal number you can cheat and still be within the motors safe operating area. Its true application has several other factors that change the output of the motor and will typically still keep the motor in its designed parameters under realistic operating loads despite the frequency being different.

If you however raise the input voltage to keep the V/F ratio the same as the lower frequency then the motor will be capable of producing more output power than its original design specs because the torque and RPM's it can hold that torque at are higher but the inherent internal loss limits are being exceeded and will burn it up.

Real life induction motor physics have several other factors in the equation than just the V/F ratio. There is the current to slip rate and counter EMF at different loads to factor in as well. Plus the changes in energy transfer that may be a gain in one part can also be counter balanced by a loss in the other part.

V/F ratio only calculations don't factor in all the other primary variables. A good hands on real world working experience engineer would have known that!

And he/she would also know its sort of a one way street when changing the frequency. Going from a 60 Hz designed motor to a 50 Hz power source has typically far more problems. Ask an experienced service tech or maintenance person that works around mixed frequency equipment. 50 Hz equipment on 60 Hz power typically works with little problem. 60 Hz equipment on 50Hz power has far more problems.

Been there done that and thats why I know the real world problems. If your wondering how I know this I had a very good electric motors lab when I went to vocational college right out of high school! These principals I mentioned were in fact standard hands on lab demonstrations. When I got into the real working world these type of situations were fairly common for me so I received loads of hands on experience dealing with them.

However my university level EE education didn't even cover how to read a motors specs plate to even find its voltage and frequency ratings.

Maybe some engineering people should have taken more hands on course work and less book work so as to know a bit more as to how the real world works even when the theory numbers say other wise!

"Theory says theory and reality are the same. Reality says different!"

Start screaming buddy! (all meant in good humor and good will though)

What the OP does is his business in the end. I don't get paid and don't really care what happens any way. I just like having a good argument now and then to keep me sharp!

Generally, it is not recommended to use a pump motor rated at 380V/50hz with voltage of 480V/60. The voltage is over the permissible voltage level of +10% of the voltage range. The pump will goes up in smoke. Bringing down the voltage from 480V/60 hz to 380V/60hz by using a step down transformer is acceptable as what Mr. tcmtech had recommended but on the other hand as what Mr.JRaef had pointed out that, running a pump motor rated at 50 hz with 60 hz will increase the speed of the pump motor. Instead of your pump motor running at 1500 RPM with 50hz, the speed will increase to 1800 RPM with 60 hz frequency but your motor torque will reduces. When you change the V/Hz ratio, you change the saturation point of the rotor and stator.

But then again, if the application is not a very critical application, like just pumping out the water from the pit, I should said, it shouldn't be any problem to use 60 hz to run a 50 hz motor with the step down voltage from 480V to 380V. For critical process control like water flow it is very importance to get the correct water pump motor power with the correct application voltage. Power requirements on a centrifugal pump vary by the cube of the flow, and flow changes proportionally to speed. This is very true.

Both gentleman's has their point of view but it all depends the actual application of individual needs. Some application are not that importance that they can tolerance some losses of power but some application cannot tolerance any losses.